This innovation is also sometimes called rotational gravity or centrifugal gravity.

One of the most perplexing problems involved with space travel is weightlessness. Prolonged exposure to microgravity can lead to decreased bone density, fluid loss, osteoporosis, and a host of other complications. Dietary supplements and rigorous daily exercise can mitigate many of the effects for a while, but can’t arrest them completely.

The solution to this problem has been known for almost a century: artificial gravity through centripetal acceleration. In other words, simulating gravity by rotating a body fast enough to provide a continuous acceleration force along the inner edge of its outer rim. Just like on whirling carnival rides, spinning constructs exert a centrifugal forcea that feel indistinguishable from true gravity.

Early in the space flight era, extensive studies were carried out by NASA and the US Air Force on the feasibility and comfort factors of rotating habitats. Visionaries such as Werner von Braun and Gerard K. O'Neill both created detailed proposals for spin gravity stations and habitats.

However, as practical human experience in space mounted, it seemed less of an immediate concern and research was halted in favor of other projects. Recently, as the possibility of very prolonged exposure to microgravity loomed on the horizon for interplanetary missions, interest in the subject is being revived. In 2011, tentative plans have been bandied about to create a small inflatable spin habitat that can be attached to the International Space Station as a practical test bed for such technology.

In science fiction, spin gravity is a long-standing motif. Fans are likely familiar with it from such on-screen properties like Babylon 5, 2001: A Space Odyssey and its sequel 2010, and the Mobile Suit Gundam anime series. It was also featured prominently in novels such as Ringworld by Larry Niven and Rendezvous with Rama by Arthur C. Clarke.

Creating a comfortable spin gravity environment can be tricky. Research has shown that the human "comfort zone" for gravity, the range people in which can live and work without adverse effects, is between 0.35 g’s and 1.0 g’s. A certain minimal rotational velocity is needed to provide this, usually starting around 6 meters per second. But the complication here is that the station cannot complete more than four revolutions per minute or else the inhabitants on board will invariable start feeling very dizzy from motion sickness. This can be a very real concern, especially if one is going to spend weeks or months on board.

It works out that with a minimal rotational velocity of six meters per second, the radius of any rotating station has to be a minimum of about 15 meters, with a human crew becoming more and more comfortable the larger the spin radius becomes. In other words, the bigger the station, the fewer rotations necessary per minute, and the easier it is to minimize motion sickness and accommodate a human crew comfortably through spin gravity.

Links in the Further Information section at the bottom of the page have the relevant equations necessary for calculating spin gravity parameters.

People used to gravity on Earth will need to make a few mental adjustments for a spin gravity environment. The sense of up and down will be decidedly different. On Earth, 'down' is always toward the center of the planet. On a spin habitat, 'down' will always be toward the outer rim of the station and 'up' will be toward the central spin axis. It would be in some ways like living on an enormous hamster wheel, where people could walk in normal gravity along the inner edge of the outer wheel rim.

The strength of the pseudo-gravity would drop off as one went 'upward' toward the central spin axis, with the exact center of the spin being in free fall. On small stations and habitats, this effect could be very pronounced, with the person going from, say, earth-normal gravity to half that in a matter of ten meters of distance or less. The exact gradient of the drop off would depend on the radius of the spin habitat and the rate of its spin, but in general small habitats will have more abrupt drop-offs than larger ones for any given distance between its rim and its center.

Coriolis effects would also be much more noticeable, especially in smaller habitats with faster rotation. The coriolis effect is the perceived curved motion of objects in a rotational frame of reference. To explain it more simply, if you throw a ball in space in microgravity, it will fly in a straight line. However, on a rotating habitat, everything on the station, including human observes, are moving in a curved line around it. So to the crew of the station, the ball will seem to curve slightly in its trajectory. Every pitch thrown on a rotating space station would be a curve ball.

On a day to day basis, this coriolis effect wold be minor and easily adjusted for. However, some tasks that require precision trajectories, like firing a gun at long range target, may take quite a bit of practice to get used to how the coriolis effect deflects a projectile's path.

Spin gravity habitats would also be under more structural stress than non-rotating ones.

Torque and progressional instability will also be factors in building a spin gravity habitat. Left on its own, a spinning habitat would eventually become unstable in the plane of its rotation. Much like a spinning coin on a table will wander over its surface randomly, a rotating station can also eventually be yanked in random directions by minor instabilities in its spin.

There are several ways of countering this effect. The most common seen in science fictions stories (including Arthur C. Clarke's 2001: A Space Odyssey) is to build another habitat of about equal size and mass on the same spin axis, but rotating in the direction opposite of the first one. Counterweights can also be used, as well as powerful stabilizing gyroscopes. If no counter-rotating habitat or counterweight or gyroscope is used, attitude thrusters placed strategically on the structure can readjust the habitat's position when needed.

Spinning habitats may or may not have a structure at their central spin axis. A wheel space station, for example, could have 'spoke' corridors placed evenly along the circumference and meeting at the center of the ring, or the wheel station may be just the rim with no such cross corridors. If present, a central structure will have very little or no simulated gravity. Because of this, some systems such as power plants are usually placed there, as the lack of simulated gravity can make maintenance much easier. Central structures are also where spaceships will usually dock, to prevent the complication of having to match the outer rim's spin.

Transitioning from the non-spinning axis hub to the spinning rim, and vice-verse, can be tricky, especially with habitats with high spin rates. Some science fiction sources have suggested using elevators which would switch from one track to another then another along a circular course, each stepped track allowing it to further 'catch up' with the spin rate of the rest of the station.

TETHERED SPIN HABITAT
Tech Level: 12

This is one or more habitats attached to a central axis with one or more high-strength tethers. If just one habitat is used, usually a counterweight is used at the other end, sometimes dead weight, sometimes cargo or a service module. If two or more habitats are used, they are usually made approximately the same size and mass to counterbalance each other.

The habitats in such a scheme would be fairly isolated form one another, and crew could only transfer between the two via spaceship or EVA.

Tethers used will likely be of the strongest possible materials, such as carbon fiber or carbon nanotube cables. Instead of a single coil or strand, meshes (like a long, thin, coiled fishing net) may be used to help prevent breakage in case of impact by micrometeoroids.

At least one scheme for a manned Mars mission called for having the crew capsule and the surface module attached by a tether and spun to produce simulated gravity for the crew. A tethered spin habitat is also seen in the novel Mercury by Ben Bova.

SPIN HABITAT PODS
Tech Level: 12

These are also sometimes called dumbbell habitats or stations.

These are usually two or more spin habitats attached by a connective corridor. The usual vision for these is two large cylindrical or spherical or box-like habitat pods, and a considerably thinner central corridor spanning the two. However, the connective corridor or spar could just as easily be as large as the pods themselves. Obviously, this configuration allows much easier access among the habitats than the tethered scheme.

The spaceship Leonov in the movie 2010 had a set of counter-rotating spin habitat pods.

RING SPIN HABITAT
Tech Level: 12

This is just what the name implies: a spin habitat built as one continuous loop. It may or may not have connecting spars or corridors crossing the ring diameter, though if connected to a larger structure (like a space station or ship) those would be necessary. The ring may consist of a string of isolated individual modules making up its circumference, but is much more commonly depicted as being a single large habitat that curves back on itself.

Spinning Ring Habitats have been the subject of serious proposals since the dawn of the space age, including detailed studies by Wernher von Braun, NASA, and the USSR/Russian Space Agency. An inflatable ring test habitat has been proposed for the International Space Station and for NASA's proposed Nautilus-X. In science fiction, ring stations have been seen in films as diverse as 2001: A Space Odyssey and Armageddon.

Though in science fiction such rings are shown to be fully rounded, they could take on other shapes, such as hexagons, octagons, and so on, with each section being of the same dimensions and mass. This may be necessary if the ring habitat is built up from pre-made modules constructed on the ground, and may be easier to construct than a fully rounded ring. However, for these types of stations, the structural stresses of the rotating habitat would be concentrated on the corners connecting the different sections.

A rotating ring can also be built into a larger static structure. The internal ring would rotate, but the structure around it would remained fixed and non-rotating, allowing the ring to be 'open' to the rest of the structure's atmosphere and environment. For ships and stations with limited space, this may be a good compromise, though it would be harder to engineer than a normal ring habitat. Such 'internal' spinning rings can be seen on the spaceships Discovery One from the movie 2001: A Space Odyssey and the Bebop in the scifi anime series Cowboy Bebop.

Ring habitats have at times been envisioned to be constructed of truly colossal size, such as the constructs featured in the article on Ringworlds in the Megastructure section. Link is at the bottom of the page.

SPHERICAL SPIN HABITAT
Tech Level: 13

This is an entire sphere set to rotate. Its spin equator would have the highest perceived gravity, and would taper off as one approached the poles. At the actual spin axis poles, the perceived gravity would be zero. Unlike a ring spin habitat, transitioning from the area of highest gravity to lowest could be as simple as walking or taking a tram car from point A to point B.

Spherical habitats are sometimes shown to be carved into the central interiors of natural bodies such as moons or asteroids, which are then set spinning, providing artificial gravity to the living space inside.

Spherical spin habitats are not seen much in science fiction, but were part of the proposal for a Bernal Sphere space colony.

CYLINDRICAL SPIN HABITAT
Tech Level: 13

A spinning cylinder maximizes the amount of surface area that can be put into a spin gravity environment. Cylindrical spin habitats are usually portrayed at being at the larger end of the space structure spectrum, being the basis of full-blown orbital colonies. They can be found discussed in great detail in Gerard K. O'Neill's landmark book on space habitats, The High Frontier, and innumerable science fiction works such as those found in the Babylon 5 TV series, the Gundam anime series, and novels such as Clarke County, Space by Allen Steele.

Like spherical spin habitats, cylindrical versions can be carved into asteroids and moons, and the whole body sent spinning to created simulated gravity on the interior.